What wheelbase are you planning?
...Is the frame a custom, or one of the usual manufacturers?
...Didn't see it anywhere in the post thus far so would like to ask: Who is the wheel supplier and what are the widths?
American Racing VN502, 15"x10" rears & 15"x5" fronts. Rear tires are M&H 30"x14.5" (12" tread) Cheater slicks with tubes. Fronts are Firestone 155/80-15
I definitely like the wheels back, (maybe that's why I'm partial to the 27) how about add 6" or so in the frame's kick-up?
I haven't a clue based from experience, as to how narrow-able the Jag rear is, (but it looks like it would be a job from hell to pull off) but if you widen the extended portion of the kickup, you'd have the structure for decent geometry for the spring/shock mounts without having to narrow.